When producing cellulose-based products, a roll press is frequently present for washing and dewatering the cellulose-based pulp. The pulp is passed between two cooperating press rolls in the roll press. The press rolls have a perforated outer surface, whereby the outer surface is permeable to liquid pressed out of the pulp. The pulp is pressed in the roll nip or press nip between the press rolls, whereby liquid is pressed out of the pulp. One example of such a roll press for dewatering pulp is disclosed in International Application No. WO 97/21868, e.g., where the central axes of the press rolls lie in substantially the same horizontal plane, and the pulp passes the press nip between the press rolls in a vertical direction, from below upwards.
International Application No. WO 03/018907 describes sealing of a press roll in a roll press having two cooperating press rolls for dewatering material suspensions.
The above-mentioned press rolls have in common that their roll bodies have a conventional structure, where the roll body comprises an axial core, such as a central drum, which can comprise a drum or an inner drum covered with a covering plate, or the like. The central drum is provided with several supporting ribs arranged longitudinally along the longitudinal extension of the central drum and evenly distributed around the circumference of the central drum. The purpose of these supporting ribs is to strengthen the structure, and also to provide channels which axially guide liquid pressed out of the pulp to the ends of the press roll. A mantle section is attached onto these supporting ribs, which mantle section comprises several circumferential frame rings arranged along the entire length of the roll body. The purpose of the frame rings is to be a supporting structure for the perforated roll plates which are positioned on the press roll, which roll plates are shrunk onto and abut the frame rings around their entire circumference, and also to allow expressed liquid, which has passed through the roll plate, to fall downwards after passage through the apertures of the roll plate. In this manner, rewetting is reduced, i.e. the liquid which, after the press nip, flows back through the roll plate and dilutes the pressed pulp, especially for roll presses where the pulp is passed through the press nip between two press rolls in a vertical direction from below upwards, such as in International Application No. WO 97/21868.
In a conventional manner, the roll plates are shrunk onto and around the mantle section. In a heated condition, the roll plates are placed around the mantle section, and subsequently, the heated roll plates are cooled down, whereby the roll plate contracts and obtains a tight abutment against the mantle section along the entire circumference. When manufacturing and repairing press rolls in a conventional manner, the short sides of the roll plate, which form a transverse joint, have been joined by welding so that a tube structure is formed, which tube structure in a heated condition is slipped onto the mantle section over one of its ends. Alternatively, upon repair of a press roll, when a roll plate is to be exchanged, the short sides can be welded together after the roll plate has been placed onto the mantle section, after which contraction of the roll plate is effected. After this, the long sides of the roll plate are welded, inter alia in order to lock the roll plate to the mantle section, i.e. to prevent movement of the roll plate in relation to the mantle section in the circumferential direction of the press roll. The welding seams are machined so that the roll plates together form an even outer surface.
U.S. Patent Publication No. 2005/0236339 A1 discloses a rotary drum filter having corrugated sheets mounted around the entire circumference of the drum filter, whereby alternating ridges and channels are formed along the longitudinal extension of the drum filter, where the ridges and channels extend along the direction of rotation of the drum filter. The drum filter comprises perforated plates which are attached to the outside of the corrugated sheets by welding the perforated plates to the tops of the ridges of the corrugated sheets. Liquid which has entered through the perforated plates is drained away by means of channels formed by the corrugated sheets.
However, there are problems associated with the welding of the roll plates, especially with regard to the welding of the longitudinal sides of the roll plates, i.e. along the circumference of the press roll. The welding seam along the longitudinal side of a roll plate is subjected to stress during operation, inter alia because of something which appears as wave motions in the roll plate, so called shearing stress, which originates from the press nip between the two press rolls of a roll press, as the one disclosed in International Application No. WO 97/21868, where the pulp is pressed together in the 8 mm press nip. As a result of this stress, the welding seam along the longitudinal side of the roll plate can sometimes crack after a certain period of operation, and this crack, or these cracks, can subsequently continue in the roll plate in an uncontrolled manner, which results in the necessity that the press roll be replaced or repaired, which in turn results in costly stoppage. In the “welding crosses”, where the welding seam of the transverse joint meets the welding seams which extend around the circumference, the risk of cracks is especially high.
U.S. Patent Publication No. 2005/0230306 A1 discloses a way to mount perforated deck segments to the frame rings of the mantle section of a press roll without welding, which deck segments have a leading edge and a trailing edge. The frame rings comprise several longitudinally extending spaced apart grooves which open outwardly. The leading edge of the deck segment is provided with a bar which is adapted to engage in the groove. The same leading edge is provided with a lip adapted to clamp the trailing edge of a deck segment between the mantle section and the leading edge lip. The leading edge of the deck segment is provided with a mounting opening and the bar is provided with an associated mounting opening, and the frame ring is provided with a hole. An attachment pin is inserted into the mounting openings of the deck segment and the bar, and into the associated frame ring hole, and attaches the bar to the mantle section.
One of the objects of the present invention is thus to provide a press roll which is more durable and more reliable in operation compared to prior art press rolls, and which avoids the above-mentioned cracks in the roll plate.